Cleaning of tray columns which have been used for rectificatively treating liquids comprising (meth)acrylic acid and/or esters thereof

ABSTRACT

In process for cleaning tray columns which are used for the purposes of rectifiying liquids comprising (meth)acrylic compounds, a basic liquid is conveyed downward through the tray column and a gas is passed through the column in countercurrent to the basic liquid at an average gas phase differential pressure over all trays of at least 0.5 mbar/tray.

The present invention relates to a process for cleaning tray columnswhich have been used for rectificatively treating liquids comprising(meth)acrylic acid and/or esters thereof by conveying a basic liquiddownward through the tray column.

In this document, (meth)acrylic acid is an abbreviated notation foracrylic acid or methacrylic acid. These and their esters are valuablestarting compounds for preparing polymers obtainable by free radicalpolymerization which find use, for example, as adhesives.

(Meth)acrylic acid itself is industrially prepared principally by.heterogeneously catalyzed gas phase oxidation of the appropriatealkenes, alkanes or the corresponding α,β-ethylenically unsaturatedaldehydes. However, this does not form pure (meth)acrylic acid. Rather,a product mixture is formed from which the (meth)acrylic acid has to beremoved. To this end, the (meth)acrylic acid is customarily absorbed ina solvent and subsequently removed via various rectification stages,optionally with the addition of azeotroping agents, from absorbents andsecondary components absorbed in addition to (meth)acrylic acid andcontained therein. Alternatively, the product gas mixture may also befractionally condensed and the (meth)acrylic acid-condensates obtainedmay be worked up rectificatively.

Esters of (meth)acrylic acid are generally prepared on the industrialscale by direct esterification of (meth)acrylic acid with alcohols, forexample alkanols, in the presence of strong acids and optionally of anazeotroping agent to remove water of esterification, or bytransesterification of (meth)acrylic esters with suitable alcohols, forexample alkanols. The target ester is likewise customarily removed fromthe product mixture predominantly rectificatively (cf., for example,EP-A 10 33 359, DE-A 19 746 688, DE-A 19 536 179 and Kirk Othmer,Encyclopedia of Chemical Technology, 4th ed., Vol. 1, pages 301-302).

Both in the case of the abovementioned rectificative removal of(meth)acrylic acid and also its esters, the rectification columns usedare generally tray columns, i.e. columns which have trays as theirinternals.

The trays of a tray column are permeable toward both the vapor rising inthe tray column and also the liquid phase refluxing in the tray column.Between the rising vapor and refluxing liquid phase, in particular onthe trays, there is heat and mass transfer as a consequence of thedisturbed equilibrium, which ultimately results in the separationdesired in the column.

A disadvantage of the rectificative removal of (meth)acrylic acid and/orits esters is that on the one hand the rectification is a thermalseparating process and on the other hand the (meth)acrylic compounds arecompounds having high boiling points and at the same time a markedtendency to free radical polymerization, in particular under the actionof heat. Typical rectification temperatures are generally above 100° C.This also applies to rectifications under reduced pressure.

The (meth)acrylic compounds are accordingly subjected in rectificationsto temperature stresses which can easily set off an undesiredpolymerization. The formation of undesired polymer fouling which inextreme cases is able to block the tray column and make it impermeableis generally the consequence. Although efforts are made in practice toprevent the undesired free radical polymerization by the addition ofsuitable polymerization inhibitors, this does not allow completeelimination of polymer formation to be achieved. In other words, thetray column generally has to be freed of polymer, i.e. cleaned, after afew weeks.

DE-A 19 746 688, DE-A 19 536 179 and EP-A 10 33 359 disclose a processfor cleaning tray columns which have been used for rectificativelytreating liquids comprising (meth)acrylic acid and/or esters thereof byconveying a basic liquid downward through the tray column.

However, a disadvantage of this procedure is that the cleaning rateobtained is not completely satisfactory.

It is an object of the present invention to provide an improved cleaningprocess.

We have found that this object is achieved by a process for cleaningtray columns which have been used for rectificatively treating liquidscomprising (meth)acrylic acid and/or esters thereof by conveying a basicliquid downward through the tray column, which comprises passing a gasthrough the tray column in countercurrent to the basic liquid in such amanner that, during the cleaning, the difference between the pressure inthe gas phase immediately below the lowermost tray of the tray columnand the pressure in the gas phase immediately above the uppermost trayof the tray column divided by the number of trays in the column is atleast 0.5, frequently from 0.5 to 6, mbar per tray.

Preference is given to passing the gas through the tray column at such arate that the abovementioned average pressure drop between twosuccessive trays is from 1 to 5 mbar and more preferably from 2 to 4mbar.

The trays in the tray column may be, for example, dual-flow trays, sievetrays, valve trays, Thormann trays, tunnel cap trays and/or bubble captrays.

The gas to be passed through the tray column in the process according tothe invention may substantially be any gas. Preference is given to usingnitrogen, air, air diluted with nitrogen and/or steam.

To convey the gas, compressors and/or vacuum pumps may be used.

The basic liquids used for the process according to the invention may beall those recommended by DE-A 19 746 688, DE-A 19 536 179 and EP-A 10 33359.

These are in particular aqueous alkali metal and/or alkaline earth metalhydroxide and/or oxide solutions, in particular the aqueous solutions ofNaOH, KOH and Ca(OH)₂. In general, the aqueous solution has a dissolvedsalt content of from 0.01 to 30% by weight, preferably from 0.5 to 10%by weight.

In an advantageous development of the invention, a substantiallypH-neutral (based on its aqueous solution) alkali metal and/or alkalineearth metal salt is added to the abovementioned basic aqueous alkalimetal solution in a ratio of >0:1 to 2:1 (weight ratio of neutral saltto hydroxide and/or oxide). For this purpose, the sulfates, acetates,oxalates, carbonates, hydrogensulfates, hydrogencarbonates and/or othersalts corresponding to the hydroxidic/oxidic compounds are particularlysuitable. Such an addition allows the dissolution behavior of the basicsolution for the process according to the invention to be furtherimproved.

According to the invention, basic polar organic solvents such as aminesor amides, preferably acetamides, more preferably monoacetamide(CH₃CONH₂) may also be used as the basic liquid instead of basic aqueoussolutions. Further basic liquids which may be used according to theinvention include monomethylacetamide (CH₃CON(CH₃)H), dimethylacetamide(CH₃CON(CH₃)₂) and dimethylformamide (HCON(CH₃)₂).

The temperatures at which the flushing according to the invention iscarried out are substantially determined by the boiling point of thebasic liquid used, since the dissolution behavior of all of the basicliquids mentioned increases with increasing temperature. The optimum usetemperature for the aqueous alkali metal and/or alkaline earth metalhydroxide solutions is from >80° C. to about 115° C. at atmosphericpressure, preferably from 90° C. to 110° C. For the amides described,the optimum use temperature in each case is from 10 to 1° C. below theboiling point of these substances. When the vapor phase of the basicflushing liquid itself is used as the gas passed in countercurrentthrough the tray column (for example steam), the use temperature isregularly the boiling point.

The process according to the invention may either be carried out atregular intervals or else after detecting a certain degree of polymerformation.

The liquids comprising (meth)acrylic acid and/or esters thereof whichhave been rectificatively treated beforehand in the tray column to becleaned according to the invention may comprise ≧20% by weight, or ≧40%by weight, or ≧60% by weight, or ≧80% by weight, or ≧90% by weight, or≧95% by weight, or ≧99% by weight of (meth)acrylic acid and/or estersthereof. The esters may be the esters of monohydroxy- and/or polyhydroxyalcohols.

In particular, the (meth)acrylic esters comprise the esters of(meth)acrylic acid with alkanols (C₁— to C₁₂—, preferably C₁— to C8—)and/or alkanediols. These are in particular methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate,tert-butyl acrylate, tert-butyl methacrylate and also 2-ethylhexylacrylate, but also the esters of dimethylaminoethanol.

Examples of polymerization inhibitors added for very substantialsuppression of polymer formation during rectification includestabilizers such as phenothiazine, hydroquinone monomethyl ether andhydroquinone.

The advantage of the process according to the invention is that thepolymer removal only entails small time demands. The final result isthus to allow a short interruption time of the rectification process andalso complete removal of the polymer deposits.

Before the process according to the invention and after carrying out theprocess according to the invention, the tray column is flushed withwater in a manner known to those skilled in the art. It is advantageousto convey a gas through the tray column in countercurrent to theflushing water in a similar manner to the process according to theinvention.

Finally, the tray column is dried and operated further.

Polymer fouling generally shows itself in the process according to theinvention by an increasing pressure drop when operating the tray column.

So that tray columns are easy to clean by the process according to theinvention, they are advantageously equipped with a flushing line. Thisfacilitates the transport to the top of the column of the basic flushingliquid which is heated, for example, in the evaporator of the column.Customarily, the flushing liquid is added via the reflux line of therectification column.

The process according to the invention may be carried out at atmosphericpressure, elevated pressure or at reduced pressure. In this document,the pressure in the gas phase “immediately” below the lowermost or abovethe uppermost tray of the tray column means that the measuring pointshould not be more than 15 cm below the lowermost and at least 25 cmabove the uppermost tray. The pressure measurement may be effected, forexample, via an open drillhole, in which a pressure transducer isconnected to the column via a wall nozzle.

It will be appreciated that the procedure according to the invention mayalso be applied to columns which have internals other than trays (forexample Raschig rings, Pall rings or structure packings) and in whichliquids comprising (meth)acrylic compounds have been rectified and/or inwhich the vapours comprising (meth)acrylic monomers have been absorbed.

It is also applicable when the column in question has become fouled withpolymer in the course of an absorption of (meth)acrylic compounds fromthe gas phase.

Overall, the advantageousness of the process according to the inventionis based on the flushing liquid passed downward through the column underthe given boundary conditions forming liquid whirlpools, in particularon the column trays, which effect the accelerated cleaning.

EXAMPLE

In a tray column (material: a stainless steel having the materialsnumber 1.4571 according to DIN standard EN 10020) of diameter 3.8 m andlength 32 m, acrylic acid was rectificatively removed via the feed lineof the column from the liquid composed as described below (feed rate=114metric tons/h).

The liquid comprised:

17% by weight of acrylic acid, 0.02% by weight of water, 0.0015% byweight of acrolein, 0.0015% by weight of acryl acrylate, 0.01% by weightof furfural, 0.027% by weight of acetic acid, 0.2% by weight ofbenzaldehyde, 0.003% by weight of propionic acid, 0.032% by weight ofmaleic anhydride, 58% by weight of diphyl, 17.0% by weight of dimethylphthalate, 3% by weight of acryloylpropionic acid and 0.02% by weight ofphenothiazine.

The tray column comprised 45 dual-flow trays (material: a stainlesssteel having the materials number 1.4571 according to DIN standard EN10020). 37 trays were above the feed point and 8 trays were below thefeed point of the acrylic acid-containing liquid. The dual-flow traysabove the feed had drillholes of diameter 25 mm and the dual-flow traysbelow the feed had drillholes of diameter 50 mm (each measuredinternally). The acrylic acid-containing liquid was separated into 99.6%by weight acrylic acid, a mixture of components having lower boilingpoints than acrylic acid and a mixture of components having higherboiling points than acrylic acid which comprised less than 0.5% byweight of acrylic acid. The homogeneous separation of the dual-flowtrays over the entire tray column was 400 mm. The temperature at the topof the column was 80° C., the pressure at the top of the column 105 mbarand the reflux ratio (ml of reflux liquid to ml of liquid withdrawal)was 1.3. The temperature at the bottom of the column was 193° C. and thepressure at the bottom of the column 230 mbar. The reflux of the columnwas stabilized with phenothiazine in such a manner that 99.6% by weightacrylic acid withdrawn via the sidestream takeoff (on tray 35, countedfrom below) comprised 250 ppm by weight of PTZ. The PTZ was addeddissolved in acrylic acid removed in this manner (1.5% by weightsolution).

After a running time of 21 days, the rectification column was shut down,emptied and then flushed for two hours with water of temperature 30° C.The flushing water was fed to the tray column via its upper reflux linein free fall and circulated by pumping via the same (300 m3/1). The usedflushing water was withdrawn from the column after the end of theflushing. The diphyl/dimethyl phthalate mixture contained therein couldbe recovered by steam distillation.

The tray column was then inspected.

In the region below the sidestream takeoff (in particular in the regionof from tray 30 to tray 35), there were about 250 kg of polymer. Thepolymer adhered both to the trays (about 60%) and also to the trayundersides (about 40%).

Flushing was then effected using a 5% by weight aqueous sodium hydroxidesolution. The aqueous sodium hydroxide solution was fed to the traycolumn via the feed line in free fall and circulated by pumping via thesame (300 m³/h). The liquid phase evaporator of the tray column wasswitched on in order to set the temperature of the sodium hydroxidesolution to from 90 to 95° C.

After one hour, the feed (in free fall) was switched to the upper refluxline and the sodium hydroxide solution circulated by pumping for afurther six hours via the same (300 m³/h). Over the entire duration ofthe sodium hydroxide solution flushing, 600 m³/h of air at ambienttemperature was fed to the column below the first tray. The average gasphase differential pressure over all trays was 2 mbar/tray. After theend of the flushing, the used flushing solution was withdrawn from thetray column. The diphyl/dimethyl phthalate mixture contained thereincould be recovered by steam distillation. The tray column was theninspected. The polymer had been removed apart from small residues (<5kg).

COMPARATIVE EXAMPLE

The procedure of the example was repeated. After the water flushing, therectification column was inspected. In the region below the sidestreamtakeoff, there were about 200 kg of polymer. The polymer adhered both tothe trays (about 50%) and also to the tray undersides (about 50%).

Flushing with sodium hydroxide solution was then effected as in theexample, except that there was no air feed. The average gas phasedifferential pressure over all trays was <<0.5 mbar/tray. After the endof the sodium hydroxide solution flushing, the rectification column wasinspected. In the region below the sidestream takeoff, there were stillabout 80 kg of polymer. The polymer was both on the trays (about 40%)and also on the tray undersides (about 60%).

The sodium hydroxide solution flushing of the example, i.e. with airfeed, was then repeated. The average gas phase differential pressureover all trays was 2.2 mbar/tray. After the second sodium hydroxidesolution flushing, the tray column was inspected again. The polymer hadbeen removed apart from small residues (<5 kg).

The pressure measurement in both the example and the comparative examplewas effected 10 cm below the lowermost or 30 cm above the uppermost trayvia an open drillhole (separation from the upper edge of the drillhole),and a pressure transducer was connected to the column via a wall nozzle.

1. A process for cleaning tray columns which have been used forrectificatively treating liquids comprising (meth)acrylic acid, estersof (meth)acrylic acid or mixtures of (meth)acrylic acid and esters of(meth)acrylic acid the process comprising conveying a basic liquiddownward through the tray column, and passing a gas through the traycolumn in countercurrent to the basic liquid, wherein during thecleaning, a difference between a pressure in the gas immediately below alowermost tray of the tray column and a pressure in the gas immediatelyabove an uppermost tray of the tray column divided by the number oftrays in the column is from 0.5 to 5 mbar per tray.
 2. The process asclaimed in claim 1, wherein, during the cleaning, the difference betweenthe pressure in the gas phase immediately above the uppermost tray ofthe tray column and the pressure in the gas phase immediately below thelowermost tray of the tray column divided by the number of trays in thecolumn is from 1 to 5 mbar per tray.
 3. The process as claimed in claim1, wherein, during the cleaning, the difference between the pressure inthe gas phase immediately above the uppermost tray of the tray columnand the pressure in the gas phase immediately below the lowermost trayof the tray column divided by the number of trays in the column is from2 to 4 mbar per tray.
 4. The process as claimed in claim 1, wherein thebasic liquid is an aqueous solution of sodium hydroxide.
 5. The processas claimed in claim 1, wherein the gas passed through the tray column incountercurrent to the basic liquid is air.
 6. The process as claimed inclaim 1, wherein the gas passed through the tray column incountercurrent to the basic liquid is selected from the group consistingof nitrogen, air, air diluted with nitrogen, steam and mixtures thereof.7. The process as claimed in claim 6, wherein a material selected fromthe group consisting of a substantially pH-neutral alkali metal salt, analkaline earth metal salt, and a mixture thereof, is added to the basicliquid.
 8. The process as claimed in claim 6, wherein the basic liquidis utilized at a temperature of from >80° C. to about 115° C.
 9. Theprocess as claimed in claim 1, wherein the basic liquid is an aqueoussolution of at least one selected from the group consisting of alkalimetal hydroxide, alkaline earth metal hydroxide, NaOH, KOH and Ca(OH)₂.10. The process as claimed in claim 1, wherein the basic liquid is abasic polar organic solvent.
 11. The process as claimed in claim 10,wherein the basic polar organic solvent is an amine, amide or a mixtureof amine and amide.